Surface antigen of the Streptococcus mutans cell has been found to be involved in the ability of the cell to form plaque on smooth surfaces. The 'a-d' polysaccharide antigen of S. mutans (strain HS6) functions as a binding site for the synthesis of dextran, the subsequent adherence of the dextran to the streptococcal cell, and the final adherence of the complex to a smooth glass surface. Antibody to the polysaccharide antigen will prevent adherence. The results show that synthesis of dextran from sucrose by enzyme which is adsorbed to the streptococcal cell is essential for adherence of the streptococci to a smooth surface. Dextran which is synthesized by S. mutans and then added to other S. mutans cells will not bring about adherence. These results indicate that the surface antigens of S. mutans play an important role in the mechanism of adherence and consequently plaque formation. It is obvious that a destruction, or an inactivation of the binding site, would probably prevent the formation of plaque. In the coming year we plan to investigate the (1) energy requirement and pH dependence of binding, (2) the inhibition of binding, and (3) the enzymatic destruction of the binding site.